Number

Element number provides the position of the piece within the sorted collection of pieces it belongs to. For instance, "Lovely Rita" can be numbered 3B (i.e. song number 3 of B-side) as regards the track listing of the original album, or 10 as regards the track listing on CD. Similarly, "III movement - Allegro" can be numbered 3 within the collection represented by all the movements that compose a sonata.
As IEEE 1599 encodes single music pieces, element number together with the contents of work_title and work_number (see below) can help reconstruct a complete music work made of many movements.